With development in business machines and progress in office automation in recent years, in the field of small-scale commercial printing, platemaking systems wherein an image is formed on a direct imaging type lithographic printing plate precursor comprising a water-resistant support having provided thereon an image receiving layer having a hydrophilic surface in a various manner to prepare an offset lithographic printing plate have been widely employed.
A conventional lithographic printing plate precursor for direct imaging type comprises a support made of paper subjected to water-resistant treatment or a plastic film having provided thereon an image accepting layer (or an image receiving layer) containing an inorganic pigment, a water-soluble resin and a water resistance imparting agent. On such a direct imaging type lithographic printing plate precursor, a lipophilic image is formed with a typewriter or by hand writing, using a lipophilic ink, or by transferring an image from an ink ribbon by heat melting with a heat transfer printer, thereby preparing a printing plate.
However, the printing plate prepared by such a method is not sufficient in mechanical strength of image areas, so that cutting easily takes place in the image areas during printing.
On the other hand, ink jet recording is a recording method low in noise and printable at a high speed, and has recently been rapidly popularized.
As such ink jet recording methods, there are proposed various systems such as a so-called electric field controlling system in which ink is discharged using electrostatic attraction, a so-called drop-on-demand system (pressure pulse system) in which ink is discharged using oscillation pressure of a piezoelectric element, and a so- called bubble (thermal) jet system in which ink is discharged using pressure generated by forming bubbles and allowing them to grow up with heating at high temperature. According to these systems, highly accurate images can be obtained.
In these ink jet recording systems, aqueous ink using water as a main solvent, and oil-based ink using an organic solvent as a main solvent are conventionally employed.
It is also known that plate making is performed using an ink jet printer on a lithographic printing plate precursor for direct imaging type described above. In this case, although aqueous ink in which water is used as a dispersion medium is employed, the aqueous ink has the problems in that blurs appear in images formed on the precursor and in that an image drawing speed is decreased because of slow drying. In order to overcome such problems, a method using oil-based ink in which a nonaqueous solvent is used as a dispersion medium is proposed as described in JP-A-54-117203 (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
However, this method is still insufficient, because blurs are actually observed in images formed by plate making and further blurs are generated in prints upon printing. Further, the number of prints obtained is limited to several hundreds at the most. Moreover, such ink has the problem of being liable to clog a nozzle for discharging minute ink droplets which make it possible to obtain images having high resolution by the plate making.
In the ink jet recording systems, ink is usually passed through a filter and discharged from a nozzle, so that abnormal discharge of ink tends to take place by clogging of the nozzle or the filter, change in fluidity of the ink with the lapse of time, or various other factors.
This abnormal discharge of ink occurs not only with an aqueous ink composition, but also with an oil-based ink composition. Various proposals for controlling the abnormal discharge of ink have been made. For example, in order to prevent the abnormal discharge of ink in case of using an oil-based ink composition, it is proposed that the viscosity and specific resistance of the ink composition is controlled as described in JP-A-49-50935, for the ink jet recording method of the electric field controlling system. It is also proposed that the dielectric constant and specific resistance of the solvent used in the ink composition are controlled as described in JP-A-53-29808.
Furthermore, as attempts to prevent clogging of nozzles caused by ordinary oil-based ink for ink jet printer, there are proposed, for example, methods in which dispersion stability of pigment particles is improved (e.g., JP-A-4-25573, JP-A-5-25413 and JP-A-5-65443) and methods in which specific compounds are incorporated into ink compositions (e.g., JP-A-3-79677, JP-A-3-64377, JP-A-4-202386 and JP-A-7-109431).
However, when these ink compositions are used for image formation of lithographic printing plate, the images formed are poor in image strength during printing, and the resulting printing plate cannot have a sufficient press life.